A truck transporting cow manure lost its load on the Lachlan Valley Way on Monday morning.
Police said they had to close the road about 15km out of Forbes after a truckload of manure was dropped on it.
Manure was spread quite thick on the road for an estimated 100 metres, a police spokesperson said, but the truck had left the scene.
Police closed the road while a water truck attended to wash it off.
They will continue inquiries into the incident.